


I've gotten it maybe twice in the past 6 months.

Himeji Castle has a chance to give 50 diamonds as a reward but the odds are really, really low. It pays out diamonds in 4 different eras - 1,000 diamonds is the last milestone if they make it to CE. You can also get 2,150 diamonds if you use the referral system to invite a player that makes a new account and plays to Contemporary Era. Wishing Wells and Fountains of Youth have a chance to pay out 50 diamonds but you need a city full (20+) to see any considerable income on a weekly basis. I'd say it's about 100 diamonds per 300 battles, on average. Diamond league has a chance to give 25 diamonds after a battle. There can be as many as 400+ diamonds on the board to win if you get the bonus 100 in the 2nd to last gold chest on level 4. Guild Expedition is the easiest way, doing all 4 levels should average 150-250 per week.
